The state of the art in aircraft propulsion using gas turbine engines can embrace moveable exhaust nozzles, to provide "vectored" engine thrust, a technique that significantly enhances aircraft maneuverability. Typically, flight stick movements made by the pilot are translated into pitch and yaw thrust vector commands that are used to command nozzle movement. The response must be fast, reliable and accurate in terms of nozzle vector angle and vector rate. Vectoring nozzles are exposed to substantial forces and temperatures from the engine, adding complexity to the task of positioning them precisely.
Following one general design philosophy, the tips of divergent flaps on an exhaust nozzle are linked to a rigid ring and anchored to the convergent throat. The ring is positioned at longitudinal and polar orientations relative to the longitudinal axis of the engine. Ring positioning is accomplished by way of actuators, and commands, sent to each actuator, cause a change in actuator extension (i.e. cause the actuator to extend or retract).
By extending all the actuators an equal amount (longitudinal motion) the ratio of the divergent area to the convergent area is controlled, optimizing engine and aircraft performance. If only one actuator is extended, nozzle vector angle is controlled, producing a turning moment on the aircraft.
The reasons to establish that design philosophy are related to complexities when accurately locating the actuators and coordinating their motion. Inaccurate actuator control can produce uncommanded vector angles, creating undesirable forces on the aircraft. In certain nozzle/actuator configurations, mislocating actuators can induce high stresses on the ring, perhaps leading to a ring failure. This can happen when, because of a high load, the number of actuators is more than the minimum number needed simply to move the nozzle.